The explained circuit can be used in schools for ringing bells using a cell phone from anywhere irrespective of the distance. Thus the use of this type of cell phone controlled remote bell can eliminate the need of running for the bell switch on every class period intervals.
The peon would be able to do his other scheduled work and ring the bell from any position inside the school premise or even from outside, in fact from any corner of the world.
Looking at the circuit diagram, we see a pretty straightforward configuration using only transistors.
Transistors T1 and T2 form a high gain looped audio amplifier. As can be seen the input signal needs to be derived from a modem cell phone's head phone socket.
A suitably selected ringtone starts playing on the cell phone when called from the users personal cell phone.
The ringtone passes through the headphone connection to the base of T1 via C1, R13.
The preamplifier consisting of T1 and T2 amplifies the signal to reasonable levels and feeds it to the T3 via C2 for further amplification.
T3 boosts the ringtone level to much higher levels, however this signal is still not powerful enough for driving a relay, therefore it is yet further amplified by T4, until finally the drive becomes just suitable for activating the relay.
The relay stays activated only until the incoming cell phone remains connected, on disconnecting the call the relay also disconnects.
The relay contacts may be integrated with a conventional GONG type of bell for the specified actions.
The modem cell phone can be of any type, however it must have the feature of assigning unique ringtones to each specific contact names as per the users preferences.
Here the owners number should be first stored/saved in the modem, then this particular contact name inside the modem should be assigned with a particular continuous type of ringtone.
Next, the default ringtone should be set to "empty".
That's it, the modem cell phone now becomes foolproof and responds to only the owners assigned number.
More than 1 number can be assigned in fact 99 numbers can be assigned which become the unique members of the modem phone book, such that the modem responds only to these numbers, it won't respond to any other unknown or wrong numbers.
Ideally a NOKIA 1280 may be used here as the modem.
INVENTED BY - "SWAGATAM"
R1 = 22k
R2 = 220 Ohms,
R3 = 100K,
R4,R6,R7 = 4K7
R5 = 1K
R13 = 100 ohms,
T1, T2,T4 = BC547
T3 = BC557,
C1 = 0.22uF
C2,C3 = 100uF/25v
L1 = 40 mH coil, example: a piezo buzzer coil will do.
diode = 1N4007
Relay = 12V/SPDT
Modem = NOKIA 1280
Charger section parts = as given in the diagram.